In the framework of Generation IV, the Commissariat à l'Energie Atomique studies different concepts of gas-cooled reactors (GCRs). The estimation of thermal-hydraulic performances in steady-state and in transient operations is of high importance for the designer of such systems. These evaluations require efficient and reliable simulation tools capable of modeling the whole reactor, including the core, the core vessel, the piping, the heat exchangers, and the turbomachinery. CATHARE2 is a thermal-hydraulic one-dimensional reference safety code developed and assessed for pressurized water reactors. It has been adapted to deal also with GCR applications. The assessment for these new applications requires cross comparisons with experimental representative data. Thus, CATHARE2 is validated against existing experimental data, in particular, the German power plant Oberhausen II data. Oberhausen II was a 50-MW(electric) direct-cycle helium turbine plant, operated by the German utility Energie Versorgung Oberhausen. This paper presents the plant, with a large emphasis on the helium power conversion unit, the modeling, and the comparison between experimental data and simulation results for both steady-state and transient cases. The agreement between the experimental data and the CATHARE results is quite satisfactory for the analyzed cases.